The invention is directed to interpretation of alternans results derived from an electrocardiogram (ECG) waveform.
Alternans, a subtle beat-to-beat change in the repeating pattern of an (ECG) waveform can be indicative of electrical instability of the heart and increased susceptibility to sudden cardiac death. Alternans results in an ABABAB . . . pattern of variation of waveform shape between successive beats in an ECG waveform. The level of variation has been found to be a useful characterization of an individual""s cardiac electrical stability, with increasing variation being indicative of decreasing stability.
While an ECG waveform typically has an amplitude measured in millivolts, an alternans pattern of variation with an amplitude on the order of a microvolt may be clinically significant. Accordingly, the alternans pattern may be too small to be detected by visual inspection of the electrocardiogram and often must be detected and quantified electronically. Such electronic detection and quantification of the alternans pattern is further complicated by the presence of noise in the ECG waveforms, as the noise may result in beat-to-beat variations that have a larger magnitude than the alternans pattern of variation.
Also, noise in the ECG waveform can mimic the presence of alternans where none exists. For example, if a patient is breathing at one half of the heart rate, the respiration may introduce a harmonic signal having the ABABAB . . . pattern of alternans. Motion that repeats with some periodicity, such as that resulting from exercise, can create noise with a similar pattern.
There are several known methods for measuring the level of alternans present in an electrocardiogram (ECG). Examples include the methods described in U.S. Pat. Nos. 4,802,491 and 5,265,617, which are incorporated by reference. Devices measuring alternans may print an alternans trend report showing the level of alternans in the individual ECG leads for the duration of the ECG recording, along with other reference signals such as measures of heart rate, noise, abnormal beats, and artifacts. An example of an alternans trend report is described in U.S. Pat. No. 5,935,082, which are incorporated by reference.
In one general aspect, interpreting alternans data includes accessing alternans data and automatically evaluating the alternans data to produce one or more interpretation parameters based on the alternans data. The interpretation parameters are used to generate interpretation results related to the alternans data. Finally, the interpretation results are made accessible for examination.
Implementations may include one or more of the following features. For example, accessing the alternans data may include receiving the alternans data from an electrocardiogram system or reading stored alternans data from a storage medium. Accessing the alternans data also may include calculating the alternans data from electrocardiogram data.
The alternans data may include alternans measures and also may include a reference signal associated with a factor that affects the quality of the alternans measures or the generation of the alternans. For example, with respect to the quality of the alternans measures, the reference signal may include a signal that masks or mimics the presence of alternans. The reference signal also may include a measure of noise that exists in the alternans data. Signals that may affect the generation of alternans include, for example, a measure of the patient""s heart rate or respiratory activity.
The interpretation parameters may include a measure of a highest heart rate in the alternans data or a highest heart rate at which sustained alternans is definitely not present. Other examples include a measure of a heart rate above which sustained alternans exists and below which sustained alternans does not exist, or an indication of the existence or nonexistence of sustained alternans.
Automatically evaluating the alternans data may include automatically evaluating a measure of alternans that is indicative of the presence of sustained alternans. For example, the measure of alternans may include a measure of a voltage or of an area associated with the alternans. Likewise, the measure of the alternans may include a measure of a power spectrum of the alternans or a dynamically estimated magnitude of the alternans, obtained, for example, by complex demodulation of the electrocardiogram. A measure of noise associated with the alternans, e.g., a measure of a standard deviation of the noise, also may be indicative of sustained alternans. Other examples include measures of a temporal duration of the alternans, of gaps in the alternans, or of a measure of the alternans based upon evaluation of time reversed alternans data.
Automatically evaluating the alternans data may include, for example, automatically using a first search to search the alternans data for sustained alternans. After using the first search, a different search also may be used to search the alternans data for sustained alternans. The different search may be used, for example, when the first search does not find sustained alternans in the alternans data or when a determination is made that the findings of the first search are suspect as a result of a poor quality of the alternans data.
Automatically evaluating the alternans data also may include evaluating the data provided by an individual electrocardiogram lead or evaluating a combination of adjacent precordial electrocardiogram leads.
The interpretation results that are generated may include the interpretation parameter and/or a clinical interpretation regarding the existence of sustained alternans in the alternans data. The clinical interpretation may, for example, positively indicate the existence of sustained alternans, negatively indicate the existence of sustained alternans, or indicate that the existence of sustained alternans is indeterminate.
Using the interpretation parameter to calculate the interpretation results may include using the interpretation parameter to traverse a decision tree to produce the interpretation results based on the alternans data. Another example includes comparing the interpretation parameter to a heart rate threshold to produce the interpretation results based on the alternans data.
The interpretation results may be made accessible for examination by, for example, graphically displaying the alternans measure, the reference signal, and the interpretation results. The interpretation results also may be made accessible by storing the alternans measure, the reference signal, and/or the interpretation results in a human or machine readable format. In any event, the alternans measure and the reference signal may be displayed using a common time axis and the interpretation results may be graphically associated to an associated feature of the alternans measure and/or the reference signal. A message describing the interpretation results also may be included.
The alternans trend report is evaluated by a trained physician, who assigns a clinical interpretation of xe2x80x9cpositive,xe2x80x9d xe2x80x9cnegativexe2x80x9d or xe2x80x9cindeterminatexe2x80x9d to the alternans result. The alternans trend data may be difficult to interpret, especially when the alternans exists in the presence of noise or abnormal ECG beats. Ultimately, the physician must exercise subjective judgment based on his or her own experience and training to determine whether the alternans is significant and sustained, and to estimate the values of Onset HR (the heart rate at the onset of sustained alternans), Max Neg. HR (the highest heart rate at which alternans is definitively not present), and other parameters. The accuracy and reliability of the interpretation of the trend data therefore varies from physician to physician as a function of experience and training. This inter-reader variability diminishes the predictive value of the alternans test and is avoided by the automatic interpretation.
These general and specific aspects may be implemented using a method, a system, or a computer program, or any combination of system, methods, and computer programs.
Other features and advantages will be apparent from the following description, including the drawings, and from the claims.